Painting the bodies of automobiles is generally conducted by applying an electrodeposition coating composition as a primer coating composition, applying an intermediate-coating composition, and then applying a topcoat coating composition.
Examples of methods of applying the topcoat coating composition include a one-coat one-bake method, in which one kind of topcoat coating composition is applied and cured with heating; a two-coat one-bake method, in which two kinds of topcoat coating compositions (a base coating composition and a clear coating composition) are used, first by applying the base coating composition, then applying the clear coating composition on top of the uncured coating composition, and simultaneously curing the two coated films with heating; etc. Among these, in the application by the two-coat one-bake method, improvement in the design properties of the coated film is achieved by the widely carried-out usage of a coloring base coating composition containing a coloring pigment as the base coating composition. Among the above-mentioned coloring pigments, dioxazine pigments are widely used as purple coloring pigments (for example, Patent Document 1).
Heretofore, solvent-type heat-curable coating compositions that comprise a combination of a base resin (a base polymer) component such as an acrylic resin, a polyester resin, an alkyd resin having a hydroxyl group, a carboxyl group or other cross-linking functional group and a curing agent such as an amino resin, a blocked or unblocked polyisocyanate compound and an epoxy compound have been widely used as the above-mentioned coloring base coating composition.
Recently, for the purposes of environmental preservation and resource conservation, the use of aqueous coating compositions has increased. For this reason, in the above-mentioned coloring base coating composition, the switch from known solvent-type heat-curable coating compositions to aqueous heat-curable coating compositions has been made.
However, when a temperature abruptly rises during the heating and curing process, a phenomenon called “popping” is generally known to occur on the cured coated film formed from such an aqueous heat-curable coating composition. Popping is a foam-like flaw of a coated film on the surface of the coated film, where a solvent remaining inside the coated film abruptly evaporates during heating and curing, or produces a bubble inside the coated film while a resin component within the coated film is solidified, whereby a portion where a bubble is produced appears to be a foam-like flaw. This popping is also referred to as a pinhole.
Since popping is produced by the abrupt evaporation of the solvent within the coated film during heating and curing, it is generally lessened by using an organic solvent having a relatively high boiling point as the solvent within the coating composition, and lowering the evaporation rate of the solvent. However, the main component of the solvent of the aqueous coating composition is water, and the amount of organic solvent having a relatively high boiling point is limited. Therefore, popping is more likely to occur in the aqueous solution than in a solvent-type coating composition.
To address such a problem, methods for suppressing popping by using a hydrophobic solvent in the aqueous coating composition have been suggested. For example, Patent Document 2 states that an aqueous coating composition containing an organic solvent with a solubility of 10 or lower in water and in an internally cross-linked resin emulsion can provide a coated film having a homogeneous appearance of the coated film and that can suppress the generation of pinholes (popping).
Patent Document 3 states that when an aqueous coating composition that mainly comprises an aqueous dispersion of the reaction product obtained by neutralizing a product of a carboxyl group-containing reaction comprising an aromatic epoxy resin and a carboxyl group-containing acrylic resin with a basic compound, and dispersing the product in a solvent mainly composed of water that contains (a) a water-miscible organic solvent comprising 30 to 70 weight % of an ether alcohol solvent having a boiling point of 175 to 240° C. and 70 to 30 weight % of alkanol having a boiling point of 140° C. or lower, and (b) a non-water miscible organic solvent comprising 20 to 80 weight % of a hydrocarbon solvent having a boiling point of 170 to 240° C. and 80 to 20 weight % of a higher alcohol solvent having a boiling point of 150 to 200° C., an aqueous coating composition having excellent film-forming properties, anti-popping properties and coating composition stability can be obtained.
However, when the above-mentioned aqueous coating composition containing the hydrophobic solvent uses the dioxazine pigment as a coloring pigment, it has problematic color variation during the storage of the coating composition.